Hydroxyl Assisted, Photoredox/Cobalt Co-catalyzed Semi-Hydrogenation and Tandem Cyclization of o-Alkynylphenols for Access to 2,3-Dihydrobenzofurans

Article abstract of DOI:10.1002/adsc.202000986

Herein, a hydroxyl assisted, photoredox/cobalt co-catalyzed semi-hydrogenation and tandem cyclization of o-alkynylphenols is developed towards direct assembly of 2,3-dihydrobenzofurans. Moderate to good yields were obtained for a range of sterically and electronically diverse 2-propynolphenols under mild conditions. Mechanistic studies demonstrated the inevitable role of the alcoholic hydroxyl group with (Z)-alkene as the real intermediate. Finally, a key low-valent cobalt catalyzed intramolecular hydroetherification of alkene is proposed. (Figure presented.).

Full text of DOI:10.1002/adsc.202000986

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DOI: 10.1002/adsc.202000986
Hydroxyl Assisted, Photoredox/Cobalt Co-catalyzed Semi-
Hydrogenation and Tandem Cyclization of o-Alkynylphenols for
Access to 2,3-Dihydrobenzofurans
Wan-Fa Tian,a Yao Zhu,a Yong-Qin He,b Mei Wang,a Xian-Rong Song,a
Jiang Bai,a and Qiang Xiaoa,*
a
Institute of Organic Chemistry, Jiangxi Science & Technology Normal University, Key Laboratory of Organic Chemistry,
Jiangxi Province, Nanchang, 330013, People’s Republic of China.
E-mail: xiaoqiang@tsinghua.org.cn
School of Pharmaceutical Science, Nanchang University, Nanchang, 330006, People’s Republic of China.
b
Manuscript received: August 18, 2020; Revised manuscript received: October 22, 2020;
Version of record online: ■■■, ■■■■
Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.202000986
Nowadays, extensive studies have been conducted in
dehydrogenative reactions by using this strategy, in
Abstract: Herein, a hydroxyl assisted, photoredox/
cobalt co-catalyzed semi-hydrogenation and tandem
which CoÀ H reacted with proton to release hydrogen
cyclization of o-alkynylphenols is developed to-
is the key procedure.[3a–c,4] On the other hand, CoÀ H
wards direct assembly of 2,3-dihydrobenzofurans.
mediated hydrocarbonation of alkenes represents an-
Moderate to good yields were obtained for a range
other important reaction pattern in photoredox/cobalt
of sterically and electronically diverse 2-propynol-
dual catalysis (Scheme 1Aa). In this respect, Rovis,[5]
phenols under mild conditions. Mechanistic studies
Maji,[6] König,[7] Collins[8] and other groups[9] have
demonstrated the inevitable role of the alcoholic
made excellent contributions. However, after extensive
hydroxyl group with (Z)-alkene as the real inter-
literatures survey, it is surprising to discover that the
mediate. Finally, a key low-valent cobalt catalyzed
photochemically generated low-valent cobalt com-
intramolecular hydroetherification of alkene is
plexes were rarely utilized in synthetic chemistry
proposed.
(Scheme 1Ab).[10] Considering that the feasible accessi-
bility and unique reactivity of low-valent Co(I)
complexes,[2a] exploration of new reaction patterns
Keywords: cyclization; cobalt catalysis; photoredox
involving photoinduced low-valent cobalt species is of
catalysis; semi-hydrogenation; 2,3-dihydrobenzofuran
great significance and remains a big challenge.
Transition metal catalyzed intramolecular addition
of oxygen-nucleophiles to CÀ C multiple bonds is a
Cobalt, as a first-row transition metal, has gained step- and atom-economic method for preparing oxy-
particular attention in recent years, not only because of gen-heterocycles. A range of transition metals (Pd, Pt,
its earth-abundant and inexpensive advantages, but Rh, Ir, Cu, Au, Zn. et. al)[11] have been used to catalyze
also because it offers possibilities of uncovering unique the transformation via protocols of either activation the
reactivity and selectivity.[1] As a consequence, various CÀ C multiple bond or activation the OÀ H bond.
valuable cobalt catalytic systems have been However, cobalt is rarely reported for achieving this
developed.[2] Among them, the rational combination of goal.[12]
photoredox catalysis with cobalt catalysis turned out to
Very recently, we disclosed a novel photoredox/
be a vital way to achieve novel and unique trans- cobalt co-catalyzed stereoselective transfer semi-hy-
formation since the oxidation state of cobalt could be drogenation of alkynes to Z-major alkenes.[13] Moti-
feasible manipulated via single electron transfer (SET) vated by the results and envisaged catalytic potential
process under mild conditions.[3]
of low-valent cobalt catalyst on OÀ H activation,[14] we
As shown in scheme 1A, the active low-valent CoI wondered if photoredox/cobalt co-catalytic system
or cobalt hydride (CoÀ H) species could generate could be used for the construction of 2,3-dihydroben-
feasibly in situ by the reduction of high-valent cobalt zofurans by using easily accessible ortho-alkynylphe-
precursors in the presence of electron donor reagent. nols as the substrate via a sequential semi-hydro-
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Table 1. Screening of optimal reaction conditions.[a]
Scheme 1. Photoredox/cobalt mediated reactions. PS=photo-
sensitizer. D=electron donor.
genation of alkynes and low-valent cobalt-mediated
intramolecular cyclization processes. Importantly, the
resulting 2,3-dihydrobenzofurans are ubiquitous struc-
tural motif in a wide range of biologically or
pharmaceutically active compounds.[15] Moreover, to
the best of our knowledge, directly accessing 2,3-
dihydrobenzofurans from 2-alkynyl phenols remains
unexplored.
[a] 1a (0.1 mmol), PS (1 mol%), Co(OAc)2·4H2O (10 mol%),
Ligand (15 mol%), i-Pr2NEt (3.0 equiv.), additive
(5.0 equiv.), solvent (2.0 mL), irradiation with blue LEDs
for 6 h, the reaction mixture was filtered by a small pad of
silica gel after irradiation and H NMR yield was reported
using Cl2CHCHCl2 as an internal standard. dppp=1,3-bis
1
Herein, we wish to report the first example of
hydroxyl assisted, photoredox/cobalt cocatalyzed semi-
hydrogenation and tandem intramolecular cyclizations
in one-pot manipulation for direct construction of 2,3-
dihydrobenzofurans from 2-propynolphenols. It’s note-
worthy that the inevitable role of the alcoholic
hydroxyl as the assistant group is demonstrated. Addi-
tionally, an unusual low-valent cobalt catalyzed intra-
molecular hydroetherification of alkene is suggested.
(diphenylphosphino) propane.
[b] H2O=10.0 equiv.
[c] isolated yield.
[d] no i-Pr2NEt.
[e] no cobalt catalyst.
[f] in dark.
efficiency. In order to accelerate the semi-hydrogena-
As our continuous efforts on exploring the reac- tion process, proton additives, such as AcOH and H2O,
tivity of 2-propynolphenol,[16] 1a was selected as were examined. Fortunately, the relative weaker proton
model substrate for examining the aforenamed hypoth- supplier of H2O could further enhance the yield to 75%
esis. The initial investigation used Ir[dF(CF3)ppy]2 (entries 11 and 12). A further mixture solvent screen-
(dtbbpy)PF6 as photosensitizer, Co(OAc)2·4H2O as ing revealed that the mixture of n-propanol and
cobalt
catalyst,
4,4’-di-tert-butyl-2,2’-dipyridyl THFcould improve the yield to 84% (entries 13 and
(dtbbpy) as ligand, i-Pr2NEt as sacrificial reagent with 14). Finally, increasing the amounts of H2O gave the
blue LED irradiation, and several solvents were firstly highest isolated yield of 85% (entry 15). Control
evaluated (Table 1, entries 1-3). To our delight, the experiments proved that all the parameters, including
desired product 2a was observed in the solvent of photosensitizer, cobalt catalyst, organic amine sacrifi-
CH3CN, THF, and n-propanol. Among which, n- cial reagent, and visible-light irradiation were essential
propanol delivered the best yield up to 65%. Other for successful transformation (entries 16–19).
photosensitizer, such as Ir[ppy]2(bpy)PF6, Ir[ppy]2
Having established the optimized reaction condi-
(dtbbpy)PF6, and 1, 2, 3, 5-tetrakis (carbazol-9-yl)-4, tions, the substrate scope for the cyclization was
6-dicyanobenzene (4CzlPN), displayed comparable investigated (Table 2). Notably, the substrates with
yields of 50%, 56%, and 60%, respectively (entries 4- neutral or electron-deficient groups, such as hydrogen,
6). Then the effect of other ligands was investigated fluoro, chloro, and ester, on the phenolic ring delivered
(entries 7–10). 2,2’-Bipyridine (bpy) induced higher the products in good to excellent yields (2a–2d, 68%–
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Table 2. Substrate scope.[a]
To demonstrate the potential synthetic application,
a gram-scale reaction of 1a was performed, which
gave 2a (1.20 g) in 79% yield (Scheme 2).
In order to shed light on the role of the hydroxyl
group, several control experiments were conducted.
Firstly, H2O18 was employed under identical condi-
tions, and no O18-cooperated product was observed
(Scheme 3, eq 1). This result proves that the hydroxyl
group is intact during the reaction process. Secondely,
the methyl protected substrate 1t was conducted under
standard conditions (Scheme 3, eq 2). Interestingly,
instead of 2t, but 2a and (E)-3 were isolated in 69%
and 19% yields, respectively. Despite the mechanism
remains obsucre, we speculate that a low-valent cobalt
catalyzed demethoxylation of allyl ether (Tsuji-Trost
type reaction) might involve in the reaction.[10c]
Further, hydroxyl-free compounds 1u and 1v were
subjected to standard conditions, no desired cyclization
products were detected (Scheme 3, eqs 3 and 4). Semi-
hydrogenated product 4 and over-reduced product 5
were both detected in the case of 1v. These observa-
tions suggest that the hydroxyl group plays a pivotal
for the success of the transformation.
To gain more mechanistic insight, several other
experiments were performed. Adding 2 equivalents of
radical scavengers, such as 2,2,6,6-tetrameth-
ylpiperidine-1-oxyl (TEMPO), 2,6-di-tert-butyl-4-
methylphenol (BHT), or 9,10-dihydroanthracene
[a] Reaction condition: 1 (0.1 mmol), Ir[dF(CF3)ppy]2(dtbbpy)
PF6 (1 mol%), Co(OAc)2·4H2O (10 mol%), bpy (15 mol%), i-
Pr2NEt (3.0 equiv.), H2O (10.0 equiv.), n-propanol/THF (v:v
4:1, 2.0 mL), irradiation with blue LEDs for 6 h, isolated
yield was reported, d.r. ratio was determined by H NMR
analysis.
1
94%). In contrast, weak electron-donating group, such (DHA) to the model reactions, the reaction efficiency
as methyl, slightly decreased the yield to 60% (2e). In
the case of chloro on C5 position, the product 2f was
obtained in 78% yield. 3,4-difluoro substituted phe-
nolic led to an excellent yield of 98% (2g). Next, the
substituents on the diphenylmethanol were screened.
Obviously, both electron- withdrawing and electron-
donating groups on the symmetric diphenylmethanol
could be well tolerated in the conditions, delivering the
corresponding products in high yield (2h–2l, 73%–
Scheme 2. Gram-scale reaction.
90%). Unsymmetric diphenylmethanols bearing elec-
tron withdrawing groups, such as fluoro and
trifluromethyl, delivered the products 2n and 2o in
66% or 72% yields, respectively. It’s obvious that the
heterocycle of thienyl was also compatible to generate
the corresponding product 2m in 88% yield.
Moreover, monoarylmethanols were also applicable
in the reaction. Various substituents, no matter
electron-donating or electron-deficient groups on the
phenyl ring, could be tolerated in the reaction, giving
the corresponding products in moderate to good yields
(2p–2r, 54%–74%). It should note that poor diaster-
eoselectivities were obtained for unsymmetric diphe-
nylmethanols or monoarylmethanols substrates (2m–
2r). Compound 1s, an example of typical dialkylme-
thanol, did not show any reaction efficiency, with the
Scheme 3. Determination the role of alcoholic hydroxyl group.
1H NMR yields using Cl2CHCHCl2 as an internal standard were
reported. PMB=para-methoxylphenyl.
observation of semi-hydrogenated products of alkenes.
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decreased slightly, indicating that free radicals may not valent cobalt precursors in photoredox/cobalt catalytic
involve in the reaction pathway (Scheme 4, eq 1). system, we deduced that a low-valent cobalt catalyst,
Moreover, monitoring of the reaction process showed such as CoI, may be responsible for the final
the apparent generation and consumption of alkene cyclization step.
intermediates ((Z)-6 and (E)-6), demonstrating that the
reaction proceeded via a sequential semi-hydrogena- were conducted (Figure S5). As shown in Figure S5,
tion/cyclization process (Figure 1, left). the experiments revealed that both i-Pr2NEt and Co
Subsequently, emission-quenching experiments
To demonstrate the real intermediate, (E)-6 and (Z)- (OAc)2·bpy could quench the excited state of Ir[dF
6 were isolated and used as the substrates, respectively. (CF3)ppy]2(dtbbpy)PF6. However, the quenching rate
When (E)-6 was conducted under standard condition, of i-Pr2NEt is much faster than the cobalt complex. In
only 28% of product 2a and 50% of (Z)-6 were addition, substrate 1a could not be an emission
obtained with 20% of starting material remained quencher of photosensitizer. These results suggest that
(Scheme 4, eq 2). When we employed (Z)-6 as the single electron transfer (SET) of the exited photo-
substrate, the reaction reached full conversion to catalyst *IrIII dominantly occurs with i-Pr2NEt.
generate a high yield of 80% for 2a (Scheme 4, eq 3).
In addition, the thermodynamic feasibility of the
Considering that (E)-alkene could be partially isomer- photoinduced electron transfer was analyzed by the
ized to (Z)-alkene via photoinduced energy transfer oxidation-reduction potentials. The reduction potential
process,[13,17] it is reasonable to speculate that (Z)-6 E*III/II of *Ir[dF(CF3)ppy]2(dtbbpy)PF6 and the
should be the actually active intermediate.
ground state potential EIII/II of Ir[dF(CF3)ppy]2(dtbbpy)
Besides, control experiments revealed that visible PF6 were reported as +1.21 V vs. SCE and À 1.37 V
light, photosensitizer, cobalt complex, and i-Pr2NEt vs. SCE, respectively.[18] Meanwhile, the reduction
were essential for the transformation of (Z)-6 to 2a potential EII/I of Co(OAc)2·bpy and the oxidation
(Table S2). To examine the impact of the light, this potential of i-Pr2NEt were determined to be À 1.18 V
reaction was also monitored for 2a with light on/off vs. SCE and +0.75 V vs SCE, respectively (see SI).
manipulation (Figure 1, right; Table S3). Continuous Obviously, *IrIII could be easily reductive quenched by
transformation after irradiation was noted, suggesting sacrificial agent i-Pr2NEt to afford IrII, and IrII is
that visible light irradiation promotes the generation of capable of transferring electron to the high-valent
active cobalt species which could further catalyze the cobalt complex Co(OAc)2·bpy.
cyclization in darkness. Since i-Pr2NEt was usually
Based on the above results, plausible reaction
used as the sacrificial reagent for reduction of high- pathways are proposed in Scheme 5. As consistent
with our recently article,[13] the photosensitizer IrIII is
firstly exited to its excited state *IrIII under visible light
irradiation, which could abstract electron from sacrifi-
cial reagent i-Pr2NEt to afford reductive photosensi-
tizer IrII and radical cation of DIPEA. Proton and
electron could be further released by the radical cation
of DIPEA. IrII is capable of reducing CoII to CoI, which
could capture a proton from the system to afford
cobalt-hydride species (CoIIIÀ H). Then, CoIIIÀ H species
could undergo a hydrometalation with alkyne to form
an alkenyl-cobalt species, followed by a protonation
process to release the semi-hydrogenated product (Z)-6
Scheme 4. Control experiments. Yields were reported as 1H
NMR yields using Cl2CHCHCl2 as an internal standard.
Figure 1. Yield/time diagram for preparation of 2a (left) and
light on/off experiments of (Z)-6 based system (right).
Scheme 5. Proposed mechanism.
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and CoIII species. The CoIII would be reduced to CoII
by iridium catalysis and thus close the Co cycle.
Subsequently, low-valent cobalt catalyzed cyclization
of ortho-alkenylphenols affording 3,4-dihydrobenzo-
furan may occur (path A). Once (Z)-6 is generated, CoI
can coordinate with the alkene and undergo oxidative
addition to the phenolic hydroxyl group to generate Co
reaction tube via syringe under Ar atmosphere. The reaction
mixture was stirred for 6 h at Wattecs Parallel Light Reactor
(Blue LED Light source, 10 W every position) at ambient
°
°
temperature (the temperature range from 35 C to 38 C).
Finally, the solvent was removed in vacuum and the residue
was purified by column chromatography on silica gel to afford
the compound 2.
III
intermediate 7, which could be directed and
stabilized by the alcoholic hydroxyl group. This
mechanism was proposed in the low-valent iridium (Ir
I) catalyzed reactions of phenols and supported by the
isolation of an IrIIIÀ H complex,[19] as well as proposed
in Co catalyzed hydrogenation reaction using H2O as
the hydrogen source.[14] Due to the higher acidic of the
phenolic hydroxyl group than alcoholic hydroxyl
group, the oxidative addition is prone to react with the
phenolic hydroxyl group.[19a,20] Subsequent migratory
insertion in 7 would result in the formation of
intermediate 8. Finally, direct reductive elimination
releases the product 3,4-dihydrobenzofuran 2a and
regenerates the CoI catalyst.
Acknowledgements
We acknowledge the National Natural Science Foundation of
China (No. 21676131 and No. 21462019), the Science
Foundation of Jiangxi Province (No. 20143ACB20012,
20202BAB213003), Education Department of Jiangxi Province
(No. GJJ190616), and Jiangxi Science & Technology Normal
University (No. 2018BSQD024, Doctor Startup Fund) for
financial support.
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In conclusion, we have developed an efficient and
convenient dual catalytic system for the preparation of
(2,3-dihydrobenzofuran-2-yl)methanol derivatives uti-
lizing 2-propynolphenols as the substrate. The trans-
formation occurred smoothly under blue LED irradi-
ation at ambient temperature with excellent tolerance
towards various functional groups and high yield was
achieved for a gram-scale reaction. Obviously, the
alcoholic hydroxyl group plays a vital role in the
success of the reaction and (Z)-alkene was the real
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Experimental Section
General Procedure for Synthesis of
2,3-Dihydrobenzofurans 2
To a Schlenk tube containing a stirring bar was added Ir[dF
(CF3)ppy]2(dtbbpy)PF6 (0.001 mmol, 1 mol%), Co(OAc)2·4H2O
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and 2-propynolphenol 1 (0.10 mmol, 1.0 equiv.). Then, i-Pr2EtN
(0.3 mmol, 3.0 equiv.), H2O (1.0 mmol, 10.0 equiv.), and
2.0 mL anhydrous n-propanol/THF (4:1) were added to the
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